High-resolution, all-reflective imaging spectrometer

ABSTRACT

An imaging spectrometer includes an all-reflective objective module that receives an image input and produces an objective module output at an exit slit, and an all-reflective collimating-and-imaging module that receives the objective module output as an objective-end input and produces a collimating-end output, wherein the collimating-and-imaging module comprises a reflective triplet. A dispersive element receives the collimating-end output and produces a dispersive-end input into the collimating-and-imaging module that is reflected through the collimating-and-imaging module to produce a spectral-image-end output. An imaging detector receives the spectral-image-end output of the collimating-and-imaging module. The objective module may be a three-mirror anastigmat having an integral corrector mirror therein, or an all-reflective, relayed optical system comprising a set of five powered mirrors whose powers sum to substantially zero. The collimating-and-imaging module may be optimized to minimize spectral smile.

This application is a continuation of application Ser. No. 10/374,911,filed Feb. 25, 2003 now U.S. Pat. No. 6,886,953, for which priority isclaimed and whose disclosure is incorporated by reference: which in turnis a continuation-in-part of application Ser. No. 10/104,424, filed Mar.22, 2002 now U.S. Pat. No. 6,902,282, for which priority is claimed andwhose disclosure is incorporated by reference.

This invention relates to an imaging spectrometer, more particularly toan all-reflective imaging spectrometer, and in some embodiments to anapproach for reducing spectral smile.

BACKGROUND OF THE INVENTION

Imaging spectrometers operable to form high-resolution images in a widerange of spectral bands are used in scientific, military, andresource-sensing applications. In one form of the imaging spectrometer,an objective optic forms an image of a scene at a slit. One portion ofthe scene is imaged by a panchromatic detector located at the imageplane. A double-pass collimating-and-imaging optic receives anotherportion of the image passed through the slit and directs it to adispersive element. The dispersed image is passed back through thecollimating-and-imaging optic to a two-dimensional hyper-spectralimaging detector array located at the plane of the slit. One dimensionof the array contains spatial information, and the other dimensioncontains spectral information. To view and spectrally analyze atwo-dimensional scene, the entire imaging spectrometer is scanned in adirection perpendicular to the slit. Existing imaging spectrometers userefractive optics, reflective optics, or a combination of the two.

While operable, the available imaging spectrometers have someshortcomings. They tend to suffer from a problem called “spectralsmile”, which is an in-plane curvature of the spectral informationprovided by the hyper-spectral imaging detector. Additionally, they tendto have a relatively narrow field of view. The narrow field of viewlimits the lateral viewing range, and the spectral smile limits theusefulness of the results for some applications. For those cases wherethe optics contains refractive elements, there is a sensitivity totemperature changes and to the effects of radiation, particularly in aspace environment.

There a need for an improved approach to an imaging spectrometer. Thepresent invention fulfills this need, and further provides relatedadvantages.

SUMMARY OF THE INVENTION

The present approach provides an imaging spectrometer. The optics of theimaging spectrometer includes only reflective elements (i.e., mirrors),achieving wide spectral coverage and avoiding the various limitations ofrefractive elements such as chromatic aberration, spectral limitations,thermal sensitivity, sensitivity to radiation, and the high cost oflenses. The panchromatic and hyper-spectral image planes are flat,facilitating the detection of the images.

In accordance with one aspect of the invention, an imaging spectrometercomprises an all-reflective imaging spectrometer optical system havingan all-reflective objective module that receives an image input andproduces an objective module output at an exit slit, wherein theobjective module is a three-mirror anastigmat having an integralcorrector mirror therein, or an all-reflective, relayed optical systemcomprising a set of five powered mirrors whose powers sum tosubstantially zero. The imaging spectrometer further includes anall-reflective collimating-and-imaging module that receives at least aportion of the objective module output as an objective-end input andproduces a collimating-end output, wherein the collimating-and-imagingmodule comprises a reflective triplet, and a dispersive element thatreceives the collimating-end output and produces a dispersive-end inputinto the collimating-and-imaging module that is reflected through thecollimating-and-imaging module to produce a spectral-image-end output.

The collimating-and-imaging module may optionally be structured tominimize spectral smile.

The dispersive element may be of any operable type, such as a reflectiongrating or a prism.

There is preferably also a panchromatic imaging detector that receivesat least a portion of the objective module output. The separation of theobjective module output may be accomplished either spatially by placingthe panchromatic imaging detector laterally adjacent to the exit slit,or spectrally using a dichroic beamsplitter. A hyper-spectral imagingdetector receives at least a portion of the spectral-image-end output ofthe collimating-and-imaging module. The panchromatic imaging detectorand the hyper-spectral imaging detector are located on opposite sides ofan image plane coincident with the exit slit. The image plane, andthence the panchromatic imaging detector and the hyper-spectral imagingdetector, are preferably substantially flat.

In these configurations, there are excellent pupil matchingcharacteristics from the entrance pupil of the objective module, to thereimaged stop at the exit pupil of the objective module (which is alsothe entrance pupil of the collimating-and-imaging module), and on to thedispersive element. The objective module and the collimating-and-imagingmodule are able to interface at an image plane that may have, by virtueof the characteristics of the objective module, significant non-normalincidence angles of the f-cones.

The objective-module foreoptics is diffraction limited at visiblewavelengths for apertures of about one-half meter or more, and can covercross-scan fields of view of about 3 degrees or more. Thecollimating-and-imaging module rear optics covers a reimaged field ofview that is in the range of about 20 degrees or more, and allows for adispersive element that is considerably smaller (on the order of fromabout 1/7 to about 1/10 the size, or even smaller) than the primarymirror of the objective-module foreoptics. Consequently, thehigh-performance imaging spectrometer is of an acceptably small size incomparison to the size of the objective-module foreoptics.

Since the amount of information that an optical system can pass isproportional to the product of the aperture and field-of-view sizes, itis natural to generally expect that as the size of the desired field ofview increases to larger values, the aperture size would decrease insomewhat the same proportion. Also, since the optical resolution isproportional to the aperture size, this is equivalent to the statementthat covering a small field of view at high resolution would involveabout the same amount of data or information as covering a large fieldof view at low resolution. In the case of imaging spectrometers, a widerfield of view would be imaged through a smaller objective-moduleaperture. Since the field of view of the collimating and imaging modulecan remain at a reasonably high value of 20 degrees or more, the ratioof collimating-and-imaging module field of view to objective-modulefield of view would decrease. In the same sense, the ratio of theapertures of the collimating-and-imaging module to objective-modulewould increase from the above values to values approaching ½, 1/1, orlarger. This aperture-field product is the optical invariant that ispreserved throughout the optical system.

To further illustrate this relationship, one can assume that acollimating-and-imaging module with an aperture of 3 inches and a fieldof view of 20 degrees is acceptable for a given application. This yieldsa product of 60 inch-degrees. Any acceptable objective module with anequal product may then be consistent with this: 40 inches aperture and1.5 degrees field of view, 20 inches and 3 degrees, 10 inches and 6degrees, 5 inches and 12 degrees, 2.5 inches and 24 degrees, etc. In theabove progression, the ratio of collimating-and-imaging module field ofview to objective-module field of view is decreasing, while the ratio ofthe apertures of the collimating-and-imaging module to objective-moduleis increasing. The method and technique for selecting an appropriateoptical form for one of these objective-module examples is set forth inthe detailed description below.

In another embodiment, an imaging spectrometer comprises anall-reflective objective module that receives an image input andproduces an objective module output at an exit slit, and anall-reflective collimating-and-imaging module that receives at least aportion of the objective module output as an objective-end input andproduces a collimating-end output, wherein the collimating-and-imagingmodule comprises a reflective triplet. The imaging spectrometer furtherincludes a dispersive element that receives the collimating-end outputand produces a dispersive-end input into the collimating-and-imagingmodule that is reflected through the collimating-and-imaging module toproduce a spectral-image-end output. The two-passcollimating-and-imaging module is characterized by the presence of anintentionally introduced classical distortion, including pincushion orbarrel distortion, that is one-half of the magnitude of the spectralsmile introduced by the dispersive element.

The objective module is preferably a three-mirror anastigmat, athree-mirror anastigmat having an integral corrector mirror therein, oran all-reflective, relayed optical system comprising a set of fivepowered mirrors whose powers sum to substantially zero.

The dispersive element may be of any operable type, such as a reflectiongrating or a prism.

There is preferably also a panchromatic imaging detector that receivesat least a portion of the objective module output, and a hyper-spectralimaging detector that receives at least a portion of thespectral-image-end output of the collimating-and-imaging module. Thepanchromatic imaging detector and the hyper-spectral imaging detectorare located on opposite sides of an image plane coincident with the exitslit. The image plane, and thence the panchromatic imaging detector andthe hyper-spectral imaging detector, are preferably flat.

In this embodiment, the advantages of the prior embodiment are alsoachieved, and additionally the magnitude of the “spectral smile” isreduced or eliminated.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an imaging spectrometer;

FIG. 2 is a spectral-dimension schematic view of the imagingspectrometer using an all-reflective, relayed optical system comprisinga set of five powered mirrors whose powers sum to substantially zero inthe objective module;

FIG. 3 is a spatial-dimension schematic view of the imaging spectrometerof FIG. 2;

FIG. 4 is a spectral-dimension schematic view of the imagingspectrometer using a three-mirror anastigmat having an integralcorrector mirror in the objective module;

FIG. 5 is a spatial-dimension schematic view of the imaging spectrometerof FIG. 4; and

FIG. 6 is an example of a prescription for an optical triplet to reducespectral smile.

FIG. 7 is an example utilizing the optical prescription of FIG. 6, nowused in double-pass and in conjunction with a two-material prism todemonstrate the cancellation of the spectral smile.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts in general form an imaging spectrometer 20comprising an all-reflective imaging spectrometer optical system. Lightenergy 22 from an image input 24 of a viewed scene enters anall-reflective objective module 26, which serves as the foreoptics ofthe imaging spectrometer 20. (As used herein, “all-reflective” meansthat the optical module or element contains only reflective opticalelements such as mirrors in the optical path, and no powered refractiveoptical elements such as lenses in the optical path.) The objectivemodule 26 produces an objective module output 28 that is directed to animage plane 30, which is preferably flat. A first portion 32 of theobjective module output 28 is incident upon a panchromatic imagingdetector 34, which is also preferably flat, located at the image plane30 and facing the objective module 26. A second portion 36 of theobjective module output 28 passes through an exit slit 38 located at theimage plane 30. The second portion 36 of the objective module output 28is received by an all-reflective collimating-and-imaging module 40 as anobjective-end input 42. The collimating-and-imaging module 40, whichserves as the rear-optics of the imaging spectrometer 20, is preferablya reflective triplet. The collimating-and-imaging module 40 produces acollimating-end output 44. A dispersive element 46 receives thecollimating-end output 44 and after reflection produces a dispersive-endinput 48 that passes back into the collimating-and-imaging module 40.After being reflected back through the collimating-and-imaging module40, this dispersive-end input 48 becomes a spectral-image-end output 50.At least a portion of the spectral-image-end output 50 is incident upona hyper-spectral imaging detector 52, which is located at the imageplane 30 and faces the collimating-and-imaging module 40 so as toreceive the spectral-image-end output 50.

The output signal of the panchromatic imaging detector 34 is a broadspectral band image of the image input 24. The output signal of thehyperspectral-imaging detector yields spectral information for the sliceof the image input 24 corresponding to the exit slit 38. To obtain thepanchromatic and spectral information for the entire two-dimensionalimage input 24, the entire imaging spectrometer 20 is scanned in adirection perpendicular to the exit slit 38.

FIGS. 2–3 and 4–5 illustrate the detailed construction of the imagingspectrometer 20 for an all-reflective, relayed optical system comprisinga set of five powered mirrors whose powers sum to substantially zero(FIGS. 2–3) and a three-mirror anastigmat having an integral correctormirror therein (FIGS. 4–5). In these figures, the elements discussedpreviously have been assigned the corresponding reference numbers, andthe prior description is incorporated. In some cases, reference numbershave been omitted from some of FIGS. 2–5 for clarity.

Referring to FIGS. 2–3, a real entrance pupil 60 in the light energy 22is located before the light energy 22 enters the objective module 26. Inthis embodiment, the objective module 26 is an all-reflective, relayedoptical system comprising a set of five powered mirrors whose powers sumto substantially zero. The objective module 26 includes a first mirror62 having positive optical power; a second mirror 64 having negativeoptical power, wherein the second mirror 64 receives the beam pathreflected from the first mirror 62 and wherein an intermediate image isformed after the beam path reflects from the second mirror 64; a thirdmirror 66 having positive optical power, wherein the intermediate imageon the beam path is reflected from the third mirror 66; a fourth mirror68 having negative optical power, wherein the beam path received fromthe third mirror 66 is reflected by the fourth mirror 68; and a fifthmirror 70 having positive optical power, wherein the beam path receivedfrom the fourth mirror 68 is reflected by the fifth mirror 70 to theimage plane 30. An exit pupil 72 is the image of the entrance pupil 60.This form of the objective module 26 is particularly useful because ofits wide field of view, but it is relatively complex and must be alignedwith great care because it includes five mirrors. An operable mirrorarrangement of this type is disclosed in application Ser. No.10/104,424.

As discussed previously, at the image plane 30 the first portion of theobjective module output is incident upon the panchromatic detector (notshown in FIGS. 2–3) and the second portion passes through the exit slit38 and to the collimating-and-imaging module 40.

The collimating-and-imaging module 40 is a reflective triplet includinga positive-power primary mirror 80 that receives the objective-end input42; a negative-power secondary mirror 82 that receives the beam pathreflected from the primary mirror 80; and a positive-power tertiarymirror 84 that receives the beam path reflected from the secondarymirror 82. The beam reflected from the tertiary mirror 84 is thecollimating-end output 44 of the collimating-and-imaging module 40 thatis incident upon the dispersive element 46. A reflective triplet that isoperable in this application is disclosed in U.S. Pat. No. 4,733,955,whose disclosure is incorporated by reference.

The dispersive element 46 may be of any operable type. Preferreddispersive elements 46 are reflection gratings and prisms. In the caseof the reflection grating, the grating lines are perpendicular to theplane of the illustration in FIG. 2. In the case of the prism, the rearsurface of the prism may be coated to reflect the incident light, or aseparate flat mirror may be located immediately next to the back side ofthe prism. The position of the spectral-image-end output 50 of thedispersed light that passes back through the collimating-and-imagingmodule 40 is determined by the tilt of the reflection grating or prism.

In another embodiment, illustrated in FIGS. 4–5, the objective module 26is an all-reflective, off-axis three-mirror anastigmat having anintegral corrector mirror therein, for a total of four mirrors. Theadvantage of using this three-mirror anastigmat having the integralcorrector mirror as the objective module 26 is that it offers amoderately wide field of view, and is less complex and is lesschallenging to align than the five-mirror objective of FIGS. 2–3,because it has only four mirrors.

Structurally, the embodiment of FIGS. 4–5 is like that of FIGS. 2–3,except as will be discussed next. The prior discussion of the embodimentof FIGS. 2–3 is incorporated here. The embodiment of FIGS. 4–5 differsin that the objective module includes four mirrors, with three mirrorsarranged as a three-mirror anastigmat and the fourth mirror being acorrector mirror. Referring to FIGS. 4–5, the real entrance pupil 60 inthe light energy 22 is located before the light energy 22 enters theobjective module 26. The objective module 26 includes a three-mirroranastigmat including a first mirror 90 having positive optical power; asecond mirror 92 having negative optical power, wherein the secondmirror 92 receives the beam path reflected from the first mirror 90; anda third mirror 94 having positive optical power, wherein the thirdmirror 94 receives the beam path reflected from the second mirror 92. Anominally flat corrector mirror 96 lies in the beam path between thefirst mirror 90 and the second mirror 92 of the three-mirror anastigmatto fold the beam path between the mirrors 90 and 92. This form of theobjective module 26 is useful because of its moderately wide field ofview, but it has fewer mirrors and is therefore more readily alignedthan the five-mirror objective module 26 of FIGS. 2–3. An operablemirror arrangement of this type is disclosed in U.S. Pat. No. 5,550,672,whose disclosure is incorporated by reference. The remainder of theimaging spectrometer 20 of FIGS. 4–5 is the same as described inrelation to the embodiment of FIGS. 2–3.

The availability of a range of suitable objective modules 26 allows thedesigner of an optical imaging spectrometer to combine one of theobjective modules 26 with the appropriate detectors 34 and 52, exit slit38, collimating-and-imaging module 40, and dispersive element 46, all asdescribed above, to produce the imaging spectrometer whose imagingspectrometer optical system is operable up to various maximum fields ofview. The cost of the increased field of view is more complexity in theobjective module 26. With the range of objective modules, fields of viewof up to about 28 degrees may be obtained. If the required maximum fieldof view is not more than about 3 degrees, the objective module 26 ispreferably selected to be an on-axis three-mirror anastigmat of the typedisclosed in U.S. Pat. No. 4,101,195, whose disclosure is incorporatedby reference. If the required maximum field of view is not more thanabout 8 degrees (preferably from about 3 to about 8 degrees), theobjective module 26 is preferably selected to be an off-axisthree-mirror anastigmat of the type disclosed in U.S. Pat. No.4,265,510, whose disclosure is incorporated by reference. If therequired maximum field of view is not more than about 16 degrees(preferably from about 8 to about 16 degrees), the objective module 26is preferably selected to be the three-mirror anastigmat having anintegral corrector mirror therein, as illustrated in FIGS. 4–5 hereofand as disclosed in U.S. Pat. No. 5,550,672. If the required maximumfield of view is not more than about 28 degrees (preferably from about16 to about 28 degrees), the objective module 26 is preferably selectedto be the an all-reflective, relayed optical system comprising a set offive powered mirrors whose powers sum to substantially zero, asillustrated in FIGS. 2–3 hereof and as set forth in application Ser. No.10/104,424.

With increasing capability for the wider field of view, these mirrorarrangements for the objective modules become more complex and moredifficult to align and maintain in alignment, and therefore it is notpreferred to use an objective module with more maximum field-of-viewcapability than required. Thus, for example, if the required maximumfield of view were to be 12 degrees, it would be preferred to select thefour-mirror objective module 26 of FIGS. 4–5 hereof, rather than thefive-mirror objective module 26 of FIGS. 2–3. Even though thefive-mirror objective module 26 has a more-than-sufficient field of viewto meet the requirement, it is more complex than the four-mirrorobjective module 26.

In an embodiment of the collimating-and imaging module 40, which may beused with the embodiments of the objective module 26 shown in FIGS. 2–3or FIGS. 4–5, or any other operable type of objective module, theparameters of the reflective triplet (the mirrors 80, 82, and 84) thatform the collimating-and-imaging module 40 may be selected to minimizeor avoid entirely the “spectral smile” (also sometimes termed the“spectral frown”) effect. The spectral smile is introduced by thedispersive element 46 and is manifested by an in-plane spatial curvatureof the spectral images sensed by the hyper-spectral imaging detector 52into a bowed form. The origin of the spectral-smile effect is discussedat greater length in James and Stemberg, “The Design of OpticalSpectrometers”, Chapman and Hall, Ltd., pages 62–68 (1969).

The concern with the spectral smile is a matter of degree in therequired linearity of the hyper-spectral image. The prior art inreflective triplet optical forms with real entrance pupils as taught inU.S. Pat. No. 4,733,955, and specifically in the optical prescription ofan embodiment given therein, is suitable and desired for certain imagingapplications. Indeed, the re-creation and raytracing of the prescriptionprovided in the '955 patent shows that the 80 percent geometrical blurdiameters average only 42 microradians across the field of view, andthat the optical scene distortion is only 0.3 percent. The measure ofoptical scene distortion used here is calculated as follows at the imageplane: the tangential intercept of the chief ray at the center of thefield of view is subtracted from the tangential intercept of the chiefray at the edge of the (sagittal) field of view; and this difference isdivided by the sagittal intercept of the chief ray at the edge of the(sagittal) field of view. Both of these performance parameters aresuitably low and acceptable for certain applications.

In other cases, however, these performance parameters are notsufficient, and it is desirable to improve the hyper-spectral image tominimize or avoid the nonlinearity associated with the spectral smile.In one embodiment of the present approach, where the reflective tripletis to be used as the collimating and imaging module 40 of the imagingspectrometer 20, low geometrical aberration is still highly desired, butit is also desired that a significant amount of optical scene distortionbe intentionally introduced into the reflective triplet design tocorrect for the spectral smile that is caused by the dispersive gratingor prism. Specifically, if the grating or prism introduces X percentspectral smile in the final dispersed image, it is desired that thereflective triplet exhibit −X/2 percent optical scene distortion as anindependent imaging module, such that on the collimating and imagingpasses through the reflective triplet, the −X/2 percent optical scenedistortions of each pass add together and cancel the X percent spectralsmile.

The intentional introduction of optical scene distortion and themaintenance or improvement of geometrical aberration correction isaccomplished in the following manner and is illustrated by the opticalprescription presented below. Using a conventional optical raytracedesign and analysis software application, the designer must manipulatethe elements of the optical prescription, with particular attention tothe tilts and decenters of the three mirrors and the higher orderaspheric surface figures of the three mirrors in order to introduceoptical scene distortion and maintain good geometrical aberrationcorrection.

More specifically, the mirror parameters are optimized through the useof an optical Figure of Merit (sometimes termed the “Error Function”),which describes or promotes certain levels of geometrical aberration andscene distortion using computer raytracing optimization techniques.These techniques are discussed in reference works such as Milton Laikin,“Lens Design”, Marcel Dekker, Inc., New York, 1991, pages 1–14 (1990)and implemented in commercially available optics software codes such asthose described in the Laikin reference. The preferred approach is tofirst design the reflective triplet of the collimating-and-imagingmodule 40 to have acceptable image quality and low distortion, whilefocusing the output of the dispersive element 46 onto the hyper-spectralimage detector at the image plane 30. This basic design is thatestablished, for example, by the prescription of the '955 patent anddiscussed above. This basic design is then modified to totally orpartially cancel the spectral-smile, while retaining the first-orderparameters and imaging functionality of the basic design. Allowablevariables in the minimization of the Figure of Merit include thespacings, tilts, and decenters of the three mirrors 80, 82, and 84 inrelation to each other, and the radii and higher order aspheric surfacefigures of each of the three mirrors. Because the light passes throughthe collimating-and-imaging module twice, the correction on each pass isone-half of the spectral smile.

FIG. 6 (parts 6A and 6B) sets forth an optical prescription derived fromthese principles, for the three mirrors 80, 82, and 84 of the reflectivetriplet of the collimating-and-imaging module 40, using the conventionset forth in U.S. Pat. No. 5,550,672, This prescription has some of thesame optical characteristics as the one given in the '955 patent: 10unit aperture, 40 unit focal length, F/4.0 optical speed, and 1×20degree field of view. In contrast to the embodiment of the '955 patent,the prescription presented in FIG. 6 has 2 percent optical scenedistortion intentionally introduced (as defined above) and hasgeometrical 80 percent blur diameters that are only 22 microradians.This represents an improvement in aberration correction by a factor of2. When used as the collimating and imaging module 40 in the imagingspectrometer 20, the embodiment presented here will correct for a 4percent spectral smile introduced by the dispersive element.

FIG. 7 (parts A–C) makes use of the optical prescription of FIG. 6(parts A and B), but in a double-pass configuration in conjunction witha two-material prism to demonstrate the cancellation of the spectralsmile. Optical raytracing of the prescription of FIG. 7 shows that (1)the spectral band of 0.4 to 2.5 micrometers is dispersed over a distanceof 0.685 units at the final image plane, (2) the geometrical 80 percentblur diameters average only 30 microradians across the field of view,and (3) there is considerable reduction of the spectral smile. At thecenter wavelength of 1.55 micrometers, the curvature of the slit imageis below 0.05 percent, while at the lower and upper ends of the spectrum(0.4 and 2.5 micrometers, respectively), the residual spectral smile isabout +0.1 and −0.1 percent, respectively. This opposite-sign balancingof the residual image curvature at the wavelength extremes indicates thepractical limit in the elimination of spectral smile. Compared to the 4percent uncorrected spectral smile, the reduction to a 0.1 percentresidual is substantial.

This desired end result that minimizes spectral smile through subtledesign variations in the mirrors of the collimating-and-imaging moduleis not achievable with many optical forms, particularly not in thosedependent to a high degree on symmetry principles for their basicimaging operation. The Schmidt camera imaging optics (which also has acurved image surface) and Offner relay optics are two such exampleswhere concentric symmetry principles are the basis for imaging with lowaberrations and low distortion. Departures from those symmetryprinciples in an attempt to introduce significant optical scenedistortion lead to a rapid and highly undesirable loss of geometricalaberration correction.

Although a particular embodiment of the invention has been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

1. An imaging spectrometer, comprising: an all-reflective objectivemodule that receives an image input and produces an objective moduleoutput at an exit slit, wherein the objective module is selected fromthe group consisting of a three-mirror anastigmat having an integralcorrector mirror therein, and an all-reflective, relayed optical systemcomprising a set of five powered mirrors whose powers sum tosubstantially zero; an imaging detector which receives a first portionof the objective module output; an all-reflectivecollimating-and-imaging module that receives a second portion of theobjective module output as an objective-end input and produces acollimating-end output, wherein the collimating-and-imaging modulecomprises a reflective triplet, and wherein a mirror of thecollimating-and-imaging module includes an intentionally introduceddistortion to correct for spectral smile; and a dispersive element thatreceives the collimating-end output and produces a dispersive-end inputinto the collimating-and-imaging module that is reflected through thecollimating-and-imaging module to produce a spectral-image-end output.2. The imaging spectrometer of claim 1, wherein the objective modulecomprises the three-mirror anastigmat having the integral correctormirror.
 3. The imaging spectrometer of claim 1, wherein the objectivemodule comprises the all-reflective, relayed optical system comprisingthe set of five powered mirrors whose powers sum to substantially zero.4. The imaging spectrometer of claim 1, wherein the dispersive elementis a reflection grating.
 5. The imaging spectrometer of claim 1, whereinthe dispersive element is a prism.
 6. The imaging spectrometer of claim1, wherein the imaging detector is a panchromatic imaging detector thatreceives at least a portion of the objective module output, and whereinthe imaging spectrometer further includes a hyper-spectral imagingdetector that receives at least a portion of the spectral-image-endoutput of the collimating-and-imaging module.
 7. The imagingspectrometer of claim 1, wherein the reflective triplet comprises threemirrors, and wherein the collimating-and-imaging module includes anintentionally introduced distortion in spacings, tilts, or decenters ofthe three mirrors in relation to each other, or in the shape of at leastone of the three mirrors to correct for spectral smile.
 8. An imagingspectrometer, comprising: an all-reflective objective module thatreceives an image input and produces an objective module output at anexit slit; an all-reflective collimating-and-imaging module thatreceives at least a portion of the objective module output as anobjective-end input and produces a collimating-end output, wherein thecollimating-and-imaging module comprises a reflective triplet, wherein amirror of the collimating-and-imaging module includes an intentionallyintroduced distortion to correct for spectral smile; and a dispersiveelement that receives the collimating-end output and produces adispersive-end input into the collimating-and-imaging module that isreflected through the collimating-and-imaging module to produce aspectral-image-end output.
 9. The imaging spectrometer of claim 8,wherein the objective module comprises a three-mirror anastigmat. 10.The imaging spectrometer of claim 8, wherein the objective module isselected from the group consisting of a three-mirror anastigmat havingan integral corrector mirror therein, and an all-reflective, relayedoptical system comprising a set of five powered mirrors whose powers sumto substantially zero.
 11. The imaging spectrometer of claim 8, whereinthe objective module comprises a three-mirror anastigmat having anintegral corrector mirror.
 12. The imaging spectrometer of claim 8,wherein the objective module comprises the all-reflective, relayedoptical system comprising a set of five powered mirrors whose powers sumto substantially zero.
 13. The imaging spectrometer of claim 8, whereinthe dispersive element is a reflection grating.
 14. The imagingspectrometer of claim 8, wherein the dispersive element is a prism. 15.The imaging spectrometer of claim 8, further including a panchromaticimaging detector that receives at least a portion of the objectivemodule output, and a hyper-spectral imaging detector that receives atleast a portion of the spectral-image-end output of thecollimating-and-imaging module.
 16. The imaging spectrometer of claim15, wherein the panchromatic imaging detector and the hyper-spectralimaging detector are located on opposite sides of an image planecoincident with the exit slit.
 17. The imaging spectrometer of claim 16,wherein the panchromatic imaging detector and the hyper-spectral imagingdetector are both flat.
 18. An imaging spectrometer comprising anall-reflective imaging spectrometer optical system operable to arequired maximum field of view of up to about 28 degrees, the imagingspectrometer optical system comprising: an all-reflective objectivemodule that receives an image input and produces an objective moduleoutput at an exit slit, wherein the objective module is an on-axis threemirror anastigmat if the required maximum field of view is not more thanabout 3 degrees, an off-axis three mirror anastigmat if the requiredmaximum field of view is from about 3 to about 8 degrees, a three-mirroranastigmat having an integral corrector mirror therein if the requiredmaximum field of view is from about 8 to about 16 degrees, or anall-reflective, relayed optical system comprising a set of five poweredmirrors whose powers sum to substantially zero if the required maximumfield of view is from about 16 to about 28 degrees; an all-reflectivecollimating-and-imaging module that receives at least a portion of theobjective module output as an objective-end input and produces acollimating-end output, wherein the collimating-and-imaging modulecomprises a reflective triplet, a dispersive element that receives thecollimating-end output and produces a dispersive-end input into thecollimating-and-imaging module that is reflected through thecollimating-and-imaging module to produce a spectral-image-end output,and wherein the all-reflective collimating-and-imaging module iscorrected for the introduction of spectral smile by the dispersiveelement.
 19. The imaging spectrometer of claim 18, further including apanchromatic visible-light imaging detector that receives at least aportion of the objective module output, and a hyper-spectral imagingdetector that receives at least a portion of the spectral-image-endoutput of the collimating-and-imaging module.
 20. The imagingspectrometer of claim 18, wherein the objective module is the on-axisthree mirror anastigmat.
 21. The imaging spectrometer of claim 18,wherein the objective module is the off-axis three mirror anastigmat.22. The imaging spectrometer of claim 18, wherein the objective moduleis the three-mirror anastigmat having an integral corrector mirrortherein.
 23. The imaging spectrometer of claim 18, wherein the objectivemodule is the all-reflective, relayed optical system comprising a set offive powered mirrors whose powers sum to substantially zero.
 24. Amethod for selecting an imaging spectrometer comprising anall-reflective imaging spectrometer optical system responsive to arequired maximum field of view, comprising the steps of: determining arequired maximum field of view of up to about 28 degrees; selecting anall-reflective objective module that receives an image input andproduces an objective module output at an exit slit, wherein theobjective module is selected, responsive to the required maximum fieldof view, from the group consisting of an on-axis three mirror anastigmatif the required maximum field of view is not more than about 3 degrees,an off-axis three mirror anastigmat if the required maximum field ofview is from about 3 degrees to about 8 degrees, a three-mirroranastigmat having an integral corrector mirror therein if the requiredmaximum field of view is from about 8 degrees to about 16 degrees, andan all-reflective, relayed optical system comprising a set of fivepowered mirrors whose powers sum to substantially zero if the requiredmaximum field of view is from about 16 degrees to about 28 degrees;providing an all-reflective collimating-and-imaging module that receivesat least a portion of the objective module output as an objective-endinput and produces a collimating-end output, wherein thecollimating-and-imaging module comprises a reflective triplet, providinga dispersive element that receives the collimating-end output andproduces a dispersive-end input into the collimating-and-imaging modulethat is reflected through the collimating-and-imaging module to producea spectral-image-end output, and wherein the step of providing theall-reflective collimating-and-imaging module includes the step ofproviding the all-reflective collimating-and-imaging module correctedfor the introduction of spectral smile by the dispersive element.